1. Field of the Invention
The present invention relates to a measurement of a wavelength of light by means of an interferometer.
2. Description of the Prior Art
Conventionally, a measurement of a wavelength of light and the like has been practiced by means of an interferometer (Michelson interferometer, for example) (refer to Japanese Laid-Open Patent Publication (Kokai) No. H8-29255, for example). A principle of the measurement of the wavelength of light is described below.
First, a Michelson interferometer is configured to generate a predetermined optical path difference L. On this occasion, reference light (wavelength: λ1, known) is supplied to the Michelson interferometer. It is assumed that the number of resulting interference fringes is A. In addition, light with a wavelength to be measured (wavelength: λx, unknown) is supplied to the Michelson interferometer. It is assumed that the number of resulting interference fringes is B. Then, the wavelength λx=(A/B)λ1.
However, if the Michelson interferometer is used in the air, due to the wavelength dispersion of the refractive index of the air, λx=(A/B)λ1 does not hold in a strict sense. It is thus necessary to multiply a correction coefficient K in order to correct the influence of the refractive index of the air. Namely, λx=(A/B)Kλ1. It should be noted that the correction coefficient K is obtained by the Edlen equation (refer to Japanese Laid-Open Patent Publication (Kokai) No. H10-221020, for example). In addition, although the refractive index of the air changes according to the wavelength, the refractive index also change according to the temperature, pressure, humidity, CO2 concentration, and the like of the air. Thus, it is necessary to correctly measure the temperature, pressure, humidity, CO2 concentration, and the like of the air to correctly obtain the correction coefficient K.
However, it is difficult to correctly measure the temperature, pressure, humidity, CO2 concentration, and the like of the air. Moreover, as the wavelength λx of the light to be measured departs from the wavelength λ1 of the reference light, the error cannot be sufficiently corrected even using the correction coefficient K, and becomes larger.
Further, the correction coefficient K can only correct the influence by the refractive index of the air. The correction cannot be carried out if an alignment (positioning) of optical elements in the Michelson interferometer deviates, and an error consequently occurs in the measurement of the wavelength.